The invention relates to a hydraulic drive which comprises a plurality of hydraulic consumers, which also include a differential cylinder and which are arranged in particular on a plastics injection-molding machine.
DE 40 08 792 A1 has already disclosed a hydraulic drive with a differential cylinder and with two hydraulic machines which operate as pumps and as motors. The two hydraulic machines are used to move the piston of the differential cylinder in a stressed state, i.e. with a higher pressure level than is actually necessary in order to move the load. For this purpose, a first port of the first hydraulic machine is connected to a tank, while from the second port a pressure line leads directly to that working chamber of the differential cylinder which is remote from the piston rod. The second hydraulic machine is arranged directly between the two working chambers of the differential cylinder. Both hydraulic machines are coupled to an electric motor and can be driven at the same rotational speed by the latter. Moreover, the displacement of both hydraulic machines is adjustable. To extend the piston rod, the second hydraulic machine is set to a defined displacement, by which, assuming a constant rotational speed of the electric motor, the speed at which the piston rod is extended is determined. Then, without changing the pressures in the working chambers, the first hydraulic machine has to deliver a volume of pressure medium per unit time into the working chamber which is remote from the piston rod which precisely corresponds to the volume of the section of piston rod which has been extended per unit time. The pressure levels in the two working chambers can be changed in two ways. Firstly, the displacement of the first hydraulic machine can be increased (raising the pressure level) or reduced (lowering the pressure level) without changing the displacement of the second hydraulic machine and therefore without changing the velocity of the piston and the piston rod of the differential cylinder. Then, the two hydraulic machines together deliver a larger or smaller volume of pressure medium into that working chamber of the differential cylinder which is remote from the piston rod than the extent to which the volume of this working chamber increases, so that the pressure in this working chamber rises or falls. The pressure in the piston rod-side working chamber rises or falls accordingly. When the desired pressure level has been reached, the displacement of the first hydraulic machine is reduced or increased again. Secondly, the pressure levels can also be changed without changing the displacement of the first hydraulic machine, by reducing (raising the pressure level) or increasing (lowering the pressure level) the displacement of the second hydraulic machine. The piston of the differential cylinder then becomes slower or quicker, so that the first hydraulic machine in turn delivers more or less pressure medium than the extent to which the volume of the working chamber which is remote from the piston rod increases, and the pressure in this chamber rises or falls. By adjusting the displacement of the second hydraulic machine to the original level, the pressure change is ended and the old speed of the piston is restored.
When retracting the piston rod, it is necessary, if the pressure levels are to be constant, for the first hydraulic machine to allow a volume of oil to pass to the tank per unit time which corresponds to the volume of the piston-rod section which is retracted per unit time. To raise the pressure levels, either the displacement of the first hydraulic machine is reduced or the displacement of the second hydraulic machine is increased until the new pressure levels are reached. To lower the pressure level, either the displacement of the first machine is increased or the displacement of the second hydraulic machine is reduced until the lower pressure levels are reached.
Naturally, it is possible to adjust the two hydraulic machines in combination both when retracting the piston rod and when extending the piston rod in order to change the pressure levels.
CA 605 046 has likewise disclosed a hydraulic drive with a differential cylinder and a first hydraulic machine and a second hydraulic machine. The two hydraulic machines have a constant displacement and can be driven by an electric motor. The first hydraulic machine can, when operating as a pump, suck in pressure medium from a tank and release it into that working chamber of the differential cylinder which is remote from the piston rod. When operating as a motor, the first hydraulic machine allows pressure medium to flow out of this working chamber to the tank. The second hydraulic machine is once again arranged between the two working chambers of the differential cylinder. In this case, the two working chambers of the differential cylinder, which is used within a crane, can be blocked by means of in each case one valve which is arranged between the two ports of the second hydraulic machine and the working chambers, in order to hydraulically lock the piston and the piston rod. It is also possible for the second hydraulic machine to suck pressure medium out of the tank via a nonreturn valve and its port, which can be connected to the piston rod-side working chamber. The intention is to replace pressure medium which has been lost as a result of leakage. In this case, there is no stressed displacement of the piston of the differential cylinder, and such displacement is also not possible without additional valves when using hydraulic machines with a constant displacement.
Both the cited documents present hydraulic drives in which the two hydraulic machines are used to supply pressure medium to a single hydraulic consumer.
The invention is based on the object of constructing a hydraulic drive having features above in such a way that, without an additional hydraulic pump, at least one further hydraulic consumer can be actuated.
In a hydraulic drive of above features this object is achieved by the fact that a first port of the at least one further hydraulic consumer can be connected to the pressure line via a directional control valve, and a second port of the further hydraulic consumer can be connected to the tank. Therefore, both hydraulic machines are used, as hydraulic pumps, to supply the further hydraulic consumers with pressure medium. The second hydraulic machine sucks pressure medium out of the tank via the nonreturn valve. When the further hydraulic consumer is operating, the two working chambers of the differential cylinder are blocked by the shut-off valve(s), so that the state of the differential cylinder is not altered by operation of the further hydraulic consumer. Therefore, the two partial delivery volumes of the two hydraulic machines are available for the further consumers. This means that it is possible to use relatively small hydraulic machines.
Therefore, according to a feature of the invention, the displacement of the two hydraulic machines is adjustable. This makes it easy, as has been explained in the introduction, to allow a pressure-stressed displacement of the differential cylinder. Moreover, it is particularly expedient if the adjustable hydraulic machines can be of small nominal size. Small hydraulic machines are very quick to adjust and produce relatively little noise.
In a particularly preferred refinement at least one further hydraulic consumer can be supplied with pressure medium from both hydraulic machines via an accumulator valve which lies on the inlet side on the pressure line. The accumulator valve opens at a minimum pressure which is sufficient to adjust the delivery volume of the hydraulic machines. This ensures that the setting pressure required to adjust the hydraulic machines prevails between the ports of the hydraulic machines which are connected to the accumulator valve and the accumulator valve itself. The minimum pressure may correspond to precisely the maximum required setting pressure, but may also be selected to be higher. An accumulator valve as is advantageously used is described in DE 43 34 167 A1.
In accordance with another feature of the invention the maximum delivery volumes per unit time of the two hydraulic machines are expediently adapted to the ratio of the effective pressure faces of the differential cylinder. Therefore, for actuation of the differential cylinder, the two hydraulic machines may operate in the region of maximum adjustment, which is particularly favorable with regard to the efficiency. If the two hydraulic machines are operating at the same rotational speed, their maximum displacements, i.e. their maximum delivery volumes per revolution, are accordingly matched.